Articles with intricate shapes can be formed out of thermoplastic polymer materials. If such articles are to be connected to other units, they have been made out of non-crosslinked plastic polymers and such articles could easily be, for instance, welded to another unit. When a higher degree of mechanical stability or rigidity was desired, such articles were formed out of crosslinkable plastic polymer which was crosslinked during the molding step.
One process which is particularly advantageous for molding intricately shaped articles out of crosslinkable polymers is the rotational molding process. In this process, powder or granules of plastic polymer being crosslinkable and containing a crosslinking agent are introduced into a mold. The closed mold is then heated from the outside and rotated preferably around two axes with different rotational speeds. During this process the polymer fuses and crosslinks, thus forming a layer of crosslinked material on the inner surface of the mold. Sturdy articles with intricate shapes can economically be produced by this process. However, pipe connections or pipe fittings of crosslinked polymers have the serious drawback that they cannot be connected by standard techniques, e.g., welding or adhesive bonding, to other units such as pipes of non-crosslinked polymers or to other crosslinked sections.